Ingredients: toothpick, a basin of water, soap, sugar cube.
Operation:
1. Carefully put the toothpick on the water.
2. Put the sugar cubes in the basin, away from the toothpicks. The toothpick will move towards the sugar cube.
3. Change a basin of water, carefully put the toothpick on the water surface, and put the soap in the basin near the toothpick. Toothpicks will stay away from soap.
Explanation:
When you put the cube in the center of the basin, it will absorb some water, so there will be a small water flow in the direction of the cube, and the toothpick will move with the water flow. However, when you put soap into the basin, the surface tension at the edge of the basin is strong, so you will pull the toothpick out.
Creation: Please try. If sugar and soap were replaced by other substances, which direction would the toothpick swim?